Ionized gas, or plasma, is commonly used during the processing and fabrication of semiconductor devices. For example, plasma can be used to etch or remove material from semiconductor integrated circuit wafers, and sputter or deposit material onto semiconducting, conducting or insulating surfaces. Creating a plasma for use in manufacturing or fabrication processes, typically begins by introducing various process gases into a plasma chamber within a plasma reactor wherein the gases are in contact with a workpiece, such as an integrated circuit wafer. The molecules of the gases in the chamber are ionized into plasma by a radio frequency (rf) energy supplied to the plasma chamber from an external power source. During processing, the plasma and ionized particles contact the workpiece.
The rf energy applied to the plasma chamber introduces an electric field that accelerates electrons which then collide with individual gas molecules causing further production of electrons and ions. There are several ways to introduce an electric field within the plasma reactor. Two common types of plasma processing systems are capacitively coupled plasma processing systems and inductively coupled plasma processing systems.
FIG. 1 is an illustration of a typical capacitively coupled plasma processing system 10 for use in processing and fabrication of semiconductor devices. As shown, plasma processing system 10 includes a plasma reactor 12 having within it a plasma chamber 13. Within plasma processing chamber 13 there are two electrodes 14a and 14b which form a capacitor. Electrode 14a is coupled to ground and electrode 14b is connected to receive rf energy from a power supply 16 via a matching network 18. When power supply 16 is energized the rf energy is applied to the capacitive circuit formed between electrodes 14a and 14b. If ionizable gases are provided within plasma chamber 13 then a plasma 22 is formed when the rf energy is applied.
Since plasma processing system 10 has only one power supply 16, increasing the power of the rf signal produced by power supply 16 tends to increase both the density of the plasma (i.e., plasma density) and the direct current (dc) bias at electrode 14b and wafer 24. An increase in the dc bias usually causes a corresponding increase in the potential drop across the plasma sheath 26 which increases the energy (i.e., ion energy) of the ionized particles contacting wafer 24.
FIG. 2 is an illustration of a conventional inductively coupled plasma processing system 30 for processing and fabrication of semiconductor devices the system illustrated in FIG. 2 is of the type disclosed in U.S. Pat. Nos. 4,948,458 and 5,571,366. Inductively coupled plasma processing system 30 includes a plasma reactor 32 having a plasma chamber 33 therein. Unlike the plasma processing system 10 of FIG. 1, inductively coupled plasma processing system 30 includes two power supplies, 34 and 36, which influence the plasma created within plasma chamber 33. Power supply 34 is configured to supply an rf energy, via a match network 38, to an electrode, chuck i.e., workplace holder) 40, located within plasma reactor 32. The rf energy for power supply 34 supplies to electrode 40 develops a dc bias on a wafer 42 which is typically located on a top surface 44 of chuck 40.
Power supply 36 is configured to supply an rf energy, via a match network 46, to a coil 48 located near plasma chamber 33. Window 50, for example a ceramic plate, separates coil 48 from plasma chamber 33. Also shown, there is typically a gas supply mechanism 52 that supplies the proper chemistry required for the manufacturing process to the plasma reactor 32. A gas exhaust mechanism 54 removes particles from within plasma chamber 33 and maintains a particular pressure within plasma chamber 33. As a result, the rf energy generated by power supply 36 creates a plasma 56 provided ionizable gases are supplied to plasma chamber 33.
The control and delivery of the rf power in a plasma discharge is of fundamental importance in plasma processing. The amount of actual power in the plasma chamber greatly affects the process conditions. Significant variance in actual power delivered to the plasma chamber can unexpectedly change the anticipated contribution of other process variable parameters such as pressure, temperature and etch rate.
As illustrated in FIGS. 1 and 2, the most commonly used method of obtaining a predetermined rf power within the plasma chamber is to provide a match network within the power circuit. The match network essentially transforms the impedance (capacitive or inductive reactance) of the plasma discharge into a substantially resistive load for the power supply. The power supply (or power supplies) can then be set to a predetermined power level dependent upon the desired process parameters.
By way of example, a typical match network includes variable capacitors and/or inductors as matching components (for low to high rf frequencies), and variable cavity taps or matching stubs (for use at microwave frequencies). Match networks may be adjusted manually or automatically, however, most match networks adjust automatically to changing load conditions.
In an effort to further control the amount of rf power provided to the plasma chamber, in a typical plasma processing system the output from the power supply (or power supplies) is monitored and controlled. This usually occurs at the output of the power supply itself based in part on the assumption that the power losses in the match network are negligible.
However, rf power delivered to the plasma chamber has been found to be substantially less than the rf power supply output because of unexpected losses in, for example, the match network itself. To account for losses in the match network in capacitively coupled plasma processing systems, additional sensing and controlling circuitry has been added to the power circuit. For example, U.S. Pat. Nos. 5,175,472, 5,474,648 and 5,556,549 disclose different ways to use of an rf sensor and a controller to provide an additional feedback control loop circuit that adjusts the output of the power supply to reach a desired rf power level within the plasma chamber.
Such feedback techniques have not been used in inductively coupled plasma processing systems because it has long been believed that the two power supplies are independent of each other in that the rf power supplied to the coil controls the plasma density and the rf power supplied to the chuck controls the energy of the ions contacting the wafer (i.e., by controlling the dc bias). Therefore, by having the two power supplies it was assumed that additional control over the process was inherently provided by independently setting the outputs of the two power supplies and operating them in an open loop mode (i.e., without feedback).
However, in reality the plasma density and ion energy are not truly independent since there is coupling between the power supplied at the source and the wafer chuck. This coupling is illustrated in FIG. 3, for example, which is a graph of the dc bias versus the rf power supplied to the wafer for various rf power settings of the rf power supplied to the coil, and various gaps (i.e., 4 or 6 cm) between the top of the plasma chamber and the surface of the wafer. The data plotted in FIG. 3 were collected from a TCP.TM. 9600SE processing system available from Lam Research Corporation of Fremont, Calif. As shown, when the rf power supplied to the chuck (i.e., bottom power) increases, the magnitude of the dc bias tends to increase. However, for a given bottom power, the dc bias which is developed also depends on the rf power supplied to the coil (i.e., TCP.TM. power), and to some extent on the gap spacing 60. This coupling between the two rf power supplies is also reflected in the energy of ions contacting the wafer. Thus, while plasma density appears to be controlled exclusively by the TCP power supply, ion energy on the other hand, is not controlled exclusively by the bottom power supply.
Thus, there is a need for a method of and apparatus for more effectively controlling the plasma density and the ion energy in an inductively coupled plasma processing system.